Hmmmm... like a preserving magic, stopping the natural flow (e.g., water to ice). Originally, perhaps, a way to keep enough magic/supplies to eke out an existence through the winter months, or to jumpstart spring (during the thaw)? And, as you point out, it's not a permanent solution -- hoarded frozen things do spoil, just more slowly. (I should point out that I live in Florida, so my understanding of deep freezing is limited to my freezer.)